Does USRP down convert the received signal directly? Thank you

[sunson29]

Dear friends, 

My question could be very simple, or very hard 😞 I am using in built example: niUSRP EX PSK Rx.vi 

1. at receiver, right after driver "fetch rx data. vi", will the usrp directly down convert the received signal to base band ?  in other words, the carry frequency will be directly removed right after the fetch data.vi ?  Btw, I am talking about the complex numbers from output "data". I plot those complex number directly in Matlab, they look like a ball, so, I think they are QPSK symbols, not anymore signal with carrier frequency. 

2. If yes, (it will down convert directly), anyone can teach me how to do the up convert ? In other words, how do I give the carrier frequency back to the signal.  This is the most important question here. Could you please show me in detail by using the same example?  Thank you for your time and help! 

 I am using QPSK, USRP 2943

[AirFrantz]

Hello,

Yes the data received by the USRP Fetch is a complex baseband signal.

2. If yes, (it will down convert directly), anyone can teach me how to do the up convert ? In other words, how do I give the carrier frequency back to the signal.

Could you elaborate on what you are looking to do here?  Are you trying to look at the spectrum for the acquisition?  The acquired data only has a frequency offset, so you could plot a spectrum around the center frequency by applying this offset to the received data.

[sunson29]

Dear Jon, 

Thanks for your reply. 

Actually, I asked this question to technical support as well. She was very helpful. But allow me to ask few more question.  She told me that, it's very hard to any operation (like signal process) in pass band. And she said it needs very very large bandwidth to grab entirey of the signal. And it's much easier to work with baseband.  So, why it need large bandwidth to grab entire signal in pass band?  Could you please explain this in detail? or if there are some link to talk about this. please let me know. Thank you.

[AirFrantz]

I think she was likely referring to the fact that the USRPs have a limited bandwidth (20-160MHz), and thus can only acquire information in a band of that width around the LO frequency.  In order to analyze the entirety of a signal you'd need a device that could sample at least twice as fast as the highest frequency component of the signal of interest.

What type of signal processing do you need to perform?  What type of information are you looking to get in the acquired data?

[mathias19100]

hello,

I have a question on USRP, the block "configure signal " is essential to the proper operation of the card?

If I just want to detect a frequency of a signal without entering frequency in configure signal, it is possible?

have a nice day,

Mathias

[AirFrantz]

Mathias,

To 'detect' a frequency I assume that you'd be acquiring a spectrum and looking for spikes?  The USRP can only acquire signals within it's bandwidth around the center frequency (what is configured in that VI).  For example, if you are using a USRP with 40MHz bandwidth and set the center frequency to 1GHz, you'd be able to see signals from 1GHz-20MHz to 1GHz + 20MHz.  In order to find frequency content in a larger bandwidth, you'd need to perform multiple acquisitions with different center frequencies (i.e. sweep the center frequency through the range of interest).